Multiple Loop Reactor for Olefin Polymerization

ABSTRACT

The present invention provides a multiple loop reactor suitable for polymerizing olefins comprising at least two interconnected loop reactors, whereby said connection essentially consists of one or more transfer lines suitable for transferring polymer slurry from a reactor to another reactor and whereby said transfer line extends substantially horizontally. The invention further relates to a process for producing olefin polymers in a multiple loop react or according to the invention. The invention also relates to the use of a transfer line for transferring polymer slurry from a reactor to another reactor in a multiple loop reactor comprising at least two interconnected loop reactors, whereby said transfer line extends substantially horizontally.

FIELD OF THE INVENTION

The present invention relates to improvements in the transfer of polymerslurry from one olefin polymerization loop reactor to another olefinpolymerization loop reactor in a multiple loop reactor. More inparticular, the present invention relates to a multiple loop reactorsuitable for olefin polymerization comprising at least twointerconnected loop reactors and to a olefin polymerization processwherein polymer slurry is substantially horizontally transferred fromone loop reactor to another loop reactor through transfer lines.

BACKGROUND

Olefin polymerizations such as ethylene polymerization are frequentlycarried out using monomer, diluent and catalyst and optionallyco-monomers and hydrogen in a reactor. The polymerization is usuallyperformed under slurry conditions, wherein the product consists usuallyof solid particles and is in suspension in a diluent. The slurrycontents of the reactor are circulated continuously with a pump tomaintain efficient suspension of the polymer solid particles in theliquid diluent. The product is discharged by means of settling legs,which operate on a batch principle to recover the product Settling inthe legs is used to increase the solids concentration of the slurryfinally recovered as product slurry. The product is further dischargedto a flash tank, through flash fines, where most of the diluent andunreacted monomers are flashed off and recycled. The polymer particlesare dried, additives can be added and finally the polymer is extrudedand pelletized.

Multiple polyolefin reactors operating in series can be used for olefinpolymerizations, as is known in the prior art. Certain polymerizationprocesses comprise the use of two or several polymerization reactors,which are interconnected. A “bimodal olefin polymer” refers to an olefinpolymer that is manufactured using two reactors, which are connected toeach other in series. However, problems associated with knownpolymerization processes and apparatuses using a polymerization systemhaving two or more serially disposed polymerization reactor vessels,include inaccurate inter-reactor transfer of polymer slurry between theserially disposed reactors, while maintaining each reactor atindependently selected operating conditions. In certain cases, fewerfine particles (fines) are produced during transfer, which tend tohang-up or become trapped in transfer equipment and can even plug finesand valves. Frequent plugging causes system down time, I ost finalproduct and raw materials, and increased operating costs.

In the prior art systems, interconnected reactors have been describedwhich are disposed in substantially vertical arrangements, i.e. reactorsarranged in tandem vertical arrangement under an angle of inclinationwith respect to a horizontal axis extending from the exit of the firstreactor which is more than 45°. Such arrangements require verticalproduct transfer lines or other vertical means for transferring polymerproduct from the polymerization zone of a first reactor to thepolymerization zone of a second reactor. However, a problem associatedwith this type of configuration is that it requires the positioning ofthe reactors in a vertical arrangement, which is generally technicallylimited and results in increased fabrication costs. Also in suchconfigurations the reactors are positioned dose to one another, whichlimits their accessibility.

U.S. Pat. No. 3,3445,431 describes an apparatus for theco-polymerization of mono-1-olefins. The apparatus comprises twointerconnected reactors. Transfer of polymer product from the firstreactor to the second reactor is done by transfer of the polymer productinto an auger conveyor which carries the polymer products from the firstreactor to the wash column of the second reactor. An auger conveyor is aconveyor that has a trough or a tube in which a product moves under theaction of an endless screw or flights. Thus, for transfer of the polymerproduct from one reactor to the other reactor a mechanical,motor-driven, device is used in the apparatus described in U.S. Pat. No.3,3445,431.

In view of the above, there remains a need in the art to provide amethod and a polymerization reactor system in which operating problemsexperienced by prior art multi-reactor systems are reduced and in whichthe apparatus may be built and operated more economically than prior artsystems.

It is therefore a general object of the present invention to providemultiple, interconnected reactors that are built and operated moreeconomically than known prior art multiple reactors. Another object ofthe invention is to provide an improved method for production ofpolyolefins in general, and polyethylene in particular, in multipleinterconnected reactors. A further object of the present invention is toprovide an improved method utilizing multiple, interconnected reactors,which reduces construction and operating costs, and improves operatingperformance and operating versatility of the reactor system.

SUMMARY

The present invention relates in a first aspect to a reactor suitablefor olefin polymerization comprising at least two interconnectedreactors, whereby said connection essentially consists of one or moretransfer lines suitable for transferring polymer slurry from a reactorto another reactor and whereby said transfer line extends substantiallyhorizontally.

According to the present invention at least two reactors areinterconnected, and preferably connected in series, so that polymerproduct produced in one reactor can be transfer red to anotherpolymerization reactor for further polymerization therein. In accordancewith the present invention, the two reactors are disposed in asubstantially same horizontal plane permitting direct substantiallyhorizontal transfer of the polymer product from one to the otherpolymerization reactor. For this purpose, product transfer lines areprovided for transferring polymer product from one reactor to anotherreactor that extend substantially horizontally.

In a preferred embodiment, the transfer line extends substantiallyhorizontally from the exit of a settling leg of a reactor to the entryin another reactor. The transfer line thus connects the exit of asetting leg of a reactor with the entry of another reactor.

In another preferred embodiment, the transfer line extends substantiallyhorizontally under an angle of inclination α with respect to ahorizontal axis X-X′ which is lower than 45°, and preferably lower than40°, lower than 35°, lower than 30°, lower than 25°, lower than 20°, andmore preferably lower than 15°. In a more preferred embodiment, thetransfer line extends substantially horizontally under an angle ofinclination α with respect to a horizontal axis X-X′ which comprises 0°,1°, 2°, 3°, 4°, 5°, 6°, 7°, 8°, 9° or 10°.

As used herein the term transfer lines extending “substantiallyhorizontally” refers to the positioning of these lines with respect to ahorizontal axis X-X′, which differs from this axis X-X′ with no morethan 45°, and preferably with no more than 40°, preferably with no morethan 35°, preferably with no more than 30°, preferably with no more than25°, preferably with no more than 20°, more preferably with no more than15°, and most preferably which differs from this axis X-X′ with 0°, 1°,2°, 3°, 4°, 5°, 6°, 7°, 8°, 9° or 10°.

The term a “substantially horizontal plane”, as used herein refers to aplane that can be inclined with respect to a horizontal plane with nomore than 45°, and preferably with no more than 40°, preferably with nomore than 35°, preferably with no more than 30°, preferably with no morethan 25°, preferably with no more than 20°, more preferably with no morethan 15°, and most preferably comprising 0°, 1°, 2°, 3°, 4°, 5°, 6°, 7°,8°, 9° or 10°.

The term “substantially horizontal transfer” refers to the transfer ofpolymer slurry from one reactor to another whereby the transfer isperformed by means of a line which has an angle of inclination withrespect to a horizontal axis X-X′ which is lower than 45°, preferablytower than 40°, preferably lower than 35°, preferably lower than 30°,preferably lower than 25°, preferably tower than 20°, more preferablylower than 15°, and most preferably which comprises 0°, 1°, 2°, 3°, 4°,5°, 6°, 7°, 8°, 9° or 10°.

The “angle of inclination” as used herein is defined as the angle madeby the transfer line with respect to a horizontal axis X-X′. This angleof inclination is according to the invention lower than 45°, preferablylower than 40°, preferably lower than 35°, preferably lower than 30°,preferably lower than 25°, preferably lower than 20°, more preferablylower than 15°, and most preferably comprises 0°, 1°, 2°, 3°, 4°, 5°,6°, 7°, 8°, 9° or 10°.

In another aspect, the present invention relates to a process forproducing olefin polymers in a reactor, wherein said reactor consists ofat least two interconnected reactors, whereby said connectionessentially consists of one or more transfer fines suitable fortransferring polymer slurry from a reactor to another reactor,comprising the steps of

-   -   introducing into a reactor one or more olefin reactants,        polymerization catalysts and diluents,    -   polymerizing said one or more olefin reactants to produce        polymer slurry,    -   pumping said polymer slurry for maintaining the circulation In        said reactor,

said process further comprising one or more cycles of

-   -   allowing said polymer slurry to settle in one or more settling        legs connected to said reactor, and    -   substantially horizontally transferring said polymer slurry from        said settling legs to another reactor through said one or more        transfer lines.

In a preferred embodiment the polymer slurry is transferredsubstantially horizontally through said transfer line from the exit of asettling leg of a reactor to the entry in the other reactor through atransfer line under an angle of inclination α with respect to ahorizontal axis X-X′ which is lower than 45° and which most preferablycomprises 0°, 1°, 2°, 3°, 4°, 5°, 6°, 7°, 8°, 9° or 10°.

The present invention further relates to the use of a transfer line fortransferring polymer slurry from a reactor to another reactor in areactor comprising at least two interconnected reactors, whereby saidtransfer line extends substantially horizontally.

In a preferred embodiment the invention relates to the use of a transferline which extends substantially horizontally from the exit of asettling leg of a reactor to the entry in the other reactor under anangle of inclination α with respect to a horizontal axis X-X′ which islower than 45° and which preferably comprises 0°, 1°, 2°, 3°, 4°, 5°,6°, 7°, 8°, 9° or 10°.

The reactor and process according to the present invention presentseveral advantages over the prior art, such as allowing optimizedtransfer of a settled slurry from settling legs from one to anotherreactor in a cost-effective way.

By providing transfer lines which extend substantially horizontallybetween the interconnected reactor, two reactors can be positioned in asubstantially same horizontal plane, which has the advantage that theinstallation of the reactors is easier the reactors can be positioned ata greater distance from one another than is the case for verticallyaligned reactors. In addition, horizontal alignment of the reactorspermits to reduce construction and operating costs and improves theaccessibility of each individual reactor, which is important fromtechnical and safety point of view.

Surprisingly, it was shown that substantially horizontal transfer ofpolymer product from one to another reactor by means of the transferlines does not increase the frequency of plugging in the polymertransfer lines but provides optimal product transfer in a cost-effectiveway.

The present invention will be further disclosed in detail hereunder. Thedescription is only given by way of example and does not limit theinvention. The reference numbers relate to the hereto-annexed figures.

DETAILED DESCRIPTION OF THE FIGURES

FIG. 1 is a diagrammatic illustration of two polymerization reactors 2,3, which are connected in series in accordance with the presentinvention.

FIG. 2 is a schematic detailed representation of the connection of oneloop reactor 2 to another loop reactor 3 by means of transfer lines 16.

FIG. 3 is a schematic representation of a double loop polymerizationreactor.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to an apparatus and a process whichprovide a substantially horizontally transfer of polymer slurry from oneto the other reactor in a multiple reactor system.

The following detailed description has bee n focused on a multiple loopreactor, consisting of at least two loop reactors which areinterconnected in series. However, the present invention is notrestricted to loop reactors but is also suitable for any other type ofmultiple reactor, consisting of at least two reactors which areconnected in series.

The present invention is suitable for polymerization processes occurringin loop reactors for the production of olefin polymers consisting of thecatalytic polymerization of olefins such as C₂ to C₈ olefins in adiluent containing the monomer to be polymerized. Examples of suitablemonomers include but are not limited to those having 2 to 8 carbon atomsper molecule, such as ethylene, propylene, butylene, pentene, butadiene,isoprene, 1-hexene and the like.

The polymerization reaction can be carried out at a temperature of from50 to 120° C., preferably at temperature of from 70 to 115° C., morepreferably at temperature of from 80 to 110° C., and at a pressure offrom 20 to 100 bars, preferably at pressure of from 30 to 50 bars, morepreferably at pressure of 37 to 45 bars.

The present invention has in particular been described in terms of thepolymerization of ethylene. Suitable “ethylene polymerization” includesbut is not limited to homo-polymerization of ethylene, co-polymerizationof ethylene and a higher 1-olefin co-monomer such as butene, 1-pentene,1-hexene, 1-octene or 1-decene. More in particular, the presentinvention is described in terms of the polymerization of ethylene formanufacturing bimodal polyethylene (PE). “Bimodal PE” refers to PE thatis manufactured using two reactors, which are connected to each other inseries.

Ethylene polymerizes in a liquid diluent in the presence of a catalyst,optionally a co-catalyst, optionally a co-monomer, optionally hydrogenand optionally other additives, thereby producing polymerization slurry.

As used herein, the term “polymerization slurry” or “polymer slurry” or“slurry” means substantially a multi-phase composition including atleast polymer solid s and a liquid phase and allows for a third phase(gas) to be at least locally present in the process, the liquid phasebeing the continuous phase. The solids include catalyst and apolymerized olefin, such as polyethylene. The liquids include an inertdiluent, such as isobutane, dissolved monomer such as ethylene,co-monomer, molecular weight control agents, such as hydrogen,antistatic agents, antifouling agents, scavengers, and other processadditives.

Suitable diluents are well known in the art and include hydrocarbonsthat are inert or at least essentially inert and liquid under reactionconditions. Suitable hydrocarbons include isobutane, n-butane, propane,n-pentane, i-pentane, neopentane, isohexane and n-hexane, with isobutanebeing preferred.

Suitable catalysts are well known in the art. Examples of suitablecatalysts include but are not limited to chromium oxide such as thosesupported on silica or aluminium, organometal catalysts including thoseknown in the art as “Ziegler” or “Ziegler-Natta” catalysts, metallocenecatalysts and the like. The term “co-catalyst” as used herein refers tomaterials that can be used in conjunction with a catalyst in order toimprove the activity of the catalyst during the polymerization reaction.

In a first embodiment, the present invention relates to a multiple loopreactor suitable for olefin polymerization comprising at least twointerconnected loop reactors whereby said connection essentiallyconsists of one or more transfer lines suitable for transferring polymerslurry from a reactor to another reactor and whereby said transfer lineextends substantially horizontally. Referring now to FIG. 1, a multipleloop reactor 1 is illustrated comprising at least two polymerizationreactors 2, 3, which are interconnected in series in accordance with thepresent invention. Although, in a preferred embodiment, the inventioncomprises the use of two separate loop reactors, it will be appreciatedby those skilled in the art that the invention also may be practicedusing more than two separate reactors.

The two reactors 2, 3 may be operated in series to produce,simultaneously, the same or two different polyolefin products, dependingupon the polyolefin feed stock and catalyst provided and thetemperature, pressure and other conditions maintained inside thereactors. In the preferred embodiment of the apparatus and method ofpresent invention, the reactors are operated in series so that polymerproduct produced in one reactor is transferred to another reactor forfurther polymerization therein.

The final polyolefin product produced by the multiple loop reactor isdependent upon a wide range of variables which may be separatelyadjusted in at least two loop reactor systems, including compositions ofthe fluidizing mediums in each loop reactor, the polymerizationcatalysts, the temperatures, pressures, flow rates and other factorsmaintained in the different reactor. Thus, polymerization system of thepresent invention using the present multiple loop reactor 1 has theoperating flexibility to produce a broad spectrum of polyolefin polymerswith varying physical properties. In a preferred embodiment, loopreactors of the multiple loop reactor system are used in series toproduce a sequentially polymerized product containing specificcharacteristics.

As illustrated on FIG. 1, the multiple loop reactor comprises at leasttwo interconnected loop reactors 2,3, each consisting of a plurality ofinterconnected pipes 4 defining a flow path 8 for a polymer slurry. Inaddition, one or more se thing legs 12 connected to the pipes 4 of onereactor 2, are provided. Each of these settling legs 12 is provided witha transfer line 16 for transferring polymer slurry to another reactor 3.One or more settling legs 12 are also connected to the pipes 4 of theother reactor 3 for discharging polymer slurry from the reactor 3 into aproduct recovery zone. Furthermore, at least one pump 6 suitable formaintaining the polymer slurry in circulation in said multiple loopreactor is provided. The connection of the loop reactors 2, 3essentially consists of one or more transfer lines 16 and whereby saidtransfer line extends substantially horizontally from one reactor 2 tothe other reactor 3. In particular, the transfer line extendssubstantially horizontally under an angle of inclination α, with respectto a horizontal axis X-X′ which is lower than 45°.

Each loop reactor 2, 3 consists of a plurality of interconnected pipes4, such as a plurality of vertical pipe segments, a plurality of upperlateral pipe segments, a plurality of lower lateral pipe segments,wherein each of said vertical pipe segment is connected at an upper endthereof to one of said upper lateral pipe segment and is connected at alower end thereof to one of said lower lateral pipe segments throughelbow shaped connecting segments, thus, defining a continuous flow path8 for a polymer slurry. It will be understood that while the loopreactor 2 and 3 are illustrated with four vertical pipes, said loopreactors 2,3 may be equipped with less or more pipes, such as 4 or morepipes, for example between 4 and 20 vertical pipes. The verticalsections of the pipe segments are preferably provided with heat jackets10. Polymerization heat can be extracted by means of cooling watercirculating in these jackets of the reactor. Said reactors preferablyoperate in a liquid full mode.

The reactants including monomer ethylene, a light hydrocarbon diluentand optionally a co-monomer and hydrogen are introduced into the reactor2 by line 5. At least one reactor 2 is also fed with catalyst,optionally in conjunction with a co-catalyst or activation agent, bymeans of the conduct 11.

The polymerization slurry is maintained in circulation in the loopreactors. The polymerization slurry is directionally circulatedthroughout the loop reactor 2, 3 as illustrated by the arrows 8 by oneor more pumps, such as axial flow pump 6. The pump may be powered by anelectric motor 7. As used herein the term “pump” includes any devicefrom compressing driving, raising the pressure of a fluid, by means forexample of a piston or set of rotating impellers 9. According to thepresent invention, the pump is preferably of axial type.

In a preferred embodiment, catalysts are introduced upstream from thecirculation pump 6 via line 5, while diluent, monomer, potentialco-monomers and reaction additives are preferably introduced downstreamof the circulation pump 6 via line 11.

Each loop reactor 2, 3 is further provided with one or more settlinglegs 12 connected to the pipes 4 of the reactor 2, 3. Intermediatepolymer slurry or polymer product may be removed from the loop reactors,by continuous or periodical discharge through one or more settling legs10 along with some diluent. In the settling legs 12, the solid contentis increased with respect to its concentration in the body of the loopreactor. As illustrated in FIG. 1, polymer slurry settled in thesettling legs 12 may be removed by means of a three-way valve 15, eitherto another reactor 3, to which it is transferred by means of a transferline 16, or to the product recovery zone, for instance by means of aconduit 20.

The settling legs can be located on any segment or any elbow of saidloop reactor. In said settling legs the polymerization slurry decants sothat the slurry exiting the reactor is more concentrated in solid thanthe circulating slurry. This permits to limit the quantity of diluentthat has to be treated and re-fed to the reactor. It will be understoodthat the discharge of said settling legs may be operated in a continuousor discontinuous mode.

As illustrated in FIG. 1, one of the horizontal sections of each reactor2, 3 is provided with four settling legs 12. Although only four settlinglegs are illustrated in FIG. 1, the present invention encompasses loopreactors comprising one or more settling legs. In an embodiment of thepresent invention said loop reactor comprises 2 to 20 settling legs,preferably 4 to 12 settling legs, more preferably 6 to 10 settling legs.

The settling legs 12 are preferably provided with an isolation valve 13.These valves 13 may for example be ball valves. Under normal conditionsthese valves are open. These valves can be closed for example to isolatea settling leg from operation. Said valves 13 can be closed when thereactor pressure falls below a chosen value.

Further the settling legs can be provided with product take off ordischarge valves 14. Discharging is performed in such a way that thevolume discharged from a settling leg substantially corresponds to thevolume of polymer slurry settled in said settling leg since its previousdischarge. The discharge valve 14 may be any type of valve, which canpermit continuous or periodical discharge of polymer slurry, when it isfully open. An angle valve, or ball valve may be suitably used. Forexample, the valve may have such a structure that solid matter isprevented from accumulating or precipitating at the main body portion ofthe valve. However, the type and structure of the discharge valve can beselected by those skilled in the art as required. According anembodiment of the present invention the totality of settled slurry isdischarged at each opening of the discharge valve. When a plurality oflegs are employed, the discharge of the settled polymer slurry may bedischarged in sequence on a rotating basis for more uniform dischargingto a subsequent reactor or to a product recovery zone.

Downstream the valve 14, at the exit of the settling leg 12, a three-wayvalve 15 is provided which allows to transfer polymer slurry settled inthe settling legs, either to a product recovery zone, or to anotherreactor, by means of the transfer line 16.

Each settling leg 10 provided on one reactor 2, which is connected inseries with another reactor 3, may be provided with a transfer line 16for transfer of (intermediate) polymer slurry settled in the settlingleg 12 to the other loop reactor 3. Settling legs 12 can also beconnected to the pipes of the other loop reactor 3 for dischargingsettled polymer slurry into a product recovery zone, e.g. by means ofconduits 18. As used herein “product recovery zone” includes but is notlimited to heated or not heated flash lines, flash tank, cyclones,filters and the associated vapor recovery and solids recovery systems ortransfer lines to a following reactor and said following reactor whenseveral reactors are connected in series.

The two loop reactors 2, 3 represented in FIG. 1 are connected in seriesby means of transfer lines 16. These transfer lines comprise generallycylindrical, inter mediate product transfer lines and extendsubstantially horizontally from the exit of a settling leg of a reactorto the entry in the other reactor. In particular, the transfer lineconnects the three-way valve 15, provided at the exit of the settlingleg 12 of one reactor 2, with the entry in the other reactor 3, where apiston valve 18 is provided.

FIG. 2 is a detailed illustration of the connection of two loop reactorsby means of a transfer line 16. As illustrated in FIG. 2, the transferline extend s substantially horizontally under an inclination angle αwith respect to a horizontal axis X-X′. In a preferred embodiment, theangle α is lower than 45°, preferably lower than 40°, lower than 35°,lower than 30°, lower than 25°, lower than 20°, lower than 15° and mostpreferably corresponds to 0°, 1°, 2°, 3°, 4°, 5°, 6°, 7°, 8°, 9° or 10°.

In another embodiment, the tangent of angle α can also be defined asΔH/ΔL, whereby ΔL is the horizontal distance between the exit of asettling leg of a reactor and the entry in another reactor, and wherebyΔH is the perpendicular distance between the exit of a settling leg of areactor, in particular the three-way valve 15, and the entry in anotherreactor, in particular at the piston valve 18. In a preferredembodiment, the perpendicular distance ΔH between the exit of a settlingleg of a reactor and the entry in another reactor is smaller than thehorizontal distance ΔL between the exit of a settling leg of a reactorand the entry in another reactor.

In another preferred embodiment, the transfer line 16 can be furtherprovided with one or more means for controlling temperature, flow orpressure of the polymer slurry in said line. The means for controllingtemperature may for instance comprise a jacket In addition, diluentflushing means 17 can be provided preferably at the inlet of thetransfer line 16. Isobutane flushing means 17 enable to flush isobutanethrough the transfer line 17 and to keep the line unplugged. This isimportant when the settling leg is put out of service. One of theadvantages of such flush is that a settling leg 12 can be can put backin service after it was taken out of service.

In another embodiment, the transfer line 16 for transferring polymerslurry from one to another loop reactor is e quipped by one or morevalves, preferably piston valves 18 at the outlet of the transfer line.The piston valves 18 are capable of sealing the orifice by which thetransfer line 16 is connected to the loop reactor 3.

It is believed that positioning of loop reactors in a multiple loopreactor system in a substantially plane and the use of the substantiallyhorizontal intermediate product transfer lines enables to improveoperating characteristics for the system by facilitating theinstallation o f the reactors and by enabling the reactors to beinstalled at larger distances than is the case when using verticallyarranged reactors.

Further, such substantially horizontal configuration allows easiness oflocating the lines so that they are straight. Obstruction in thetransfer line, including curves or any other deviations from asubstantially horizontal orientation, and any unnecessary extension ofthe transfer line, may decrease the rate of transfer between the loopreactors and increases the frequency of plugging in the transfer line.

The polymerization system of the present invention minimizes theopportunity for plugging in the transfer lines by eliminating siteswhere the polyolefin polymer can begin to accumulate. Thus, in thepreferred embodiment, the transfer line is substantially horizontal inorientation and free of any bends or other obstruction. If desired,additional means may be provided in the product transfer line for aidingthe flow of polymer product between the loop reactors, e.g. vibrationhammers.

As a further safety measure in the event of polymer plugs orobstructions developing in the transfer line during operation, more thanone product transfer lines is provided between the polymerization loopreactors.

In the event that the horizontal transfer line becomes plugged orobstructed for any reason, or if a product flow rate beyond the capacityof the transfer line is desired, then polymer slurry can be withdrawnfrom one reactor through another transfer line and supplied to an otherpolymerization reactor to permit continued operation of the system. Thereactors are still working while transfer lines are out of operation.

In a preferred embodiment, more than one product transfer line is notonly used from safety point of view, but also from productivity point ofview. More than one product transfer line may be used so that the use ofsettling legs is maximized and the transfer of diluent from the firstreactor to the second reactor is minimized. In another embodiment, sometransfer lines can be put out of service, e.g. when polymerization isperformed at reduced rates and for the start-up.

Preferably, the transfer lines for transferring polymer slurry from oneto another reactor, are continuously flushed, e.g. with isobutane, inoperation or not.

In another preferred embodiment, by operation in accordance with thepresent invention, all lines, vessels, pumps, valves, etc., can be keptfree of clogging by means of flushing or purging with nitrogen ordiluent, i.e. isobutane. It is to be understood that where necessaryflushing and purging means and lines are available on the deviceaccording to the invention in order to avoid plugging, or blocking. Thisis useful for example when the reactor is to be completely emptied inorder to make an intervention on the reactor.

It is clear from the present description that numbers and dimensions ofthe different parts of the reactor relate to the size of thepolymerization reactors and can be changed in function of the reactorsizes.

In another preferred embodiment, it is to be understood that all linesor conduits applied in accordance with the present invention may beprovided, where necessary with flow measuring means.

In a preferred embodiment, the present invention can be applied on adouble loop reactor, as depicted on FIG. 3. FIG. 3 represents two singleloop reactors 100; 116, which are interconnected in series. Bothreactors 100, 116 consist of a plurality of interconnected pipes 104.The vertical sections of the pipe segments 104 are preferably providedwith heat jackets 105. Reactants are introduced into the reactors 100 byline 107. Catalyst, optionally in conjunction with a co-catalyst oractivation agent, is injected in the reactor 100 or 116 by means of theconduct 106. The polymerization slurry is directionally circulatedthroughout the loop reactors 100, 116 as illustrated by the arrows 108by one or more pumps, such as axial flow pump 101. The pumps may bepowered by an electric motor 102. The pumps may be provided with set ofrotating impellers 103. The reactors 100, 116 are further provided withone or more settling legs 109 connected to the pipes 104 of the reactors100, 116. The settling legs 109 are preferably provided with anisolation valve 110. Further the settling legs can be provided withproduct take off or discharge valves 111 or can be in directcommunication with the downstream section. Downstream the valve 111 atthe exit of the settling leg 109 of reactor 100, a transfer line 112 isprovided which allows to transfer polymer slurry settled in the settlinglegs 109 to the other reactor 116 preferably through a piston valve 115.Polymer slurry settled in the settling legs 109 of reactor 116 can beremoved by means of one or more product recovery lines 113, e.g. to aproduct recovery zone.

While the invention has been described in terms of the presentlypreferred embodiment, reasonable variations and modifications arepossible by those skilled in the art and such variations are within thescope of the described invention and the appended claims.

1-18. (canceled)
 19. A multi-reactor reactor system comprising: a first reactor, wherein the first reactor comprises a plurality of interconnected pipes defining a flow path for a polymer slurry, wherein an isolation valve is affixed to one of the plurality of interconnected pipes and a settling leg, wherein the settling leg is affixed to the isolation valve and the settling leg having an outlet end; a second reactor, wherein the second reactor comprises a plurality of interconnected pipes defining a flow path for a polymer slurry, wherein an entry valve is affixed to one of the plurality of interconnected pipes, the entry valve having an entry valve inlet end; and a transfer line having a transfer line inlet end and a transfer line outlet end, wherein the transfer line inlet end is affixed to the settling line outlet end and the transfer line outlet end is affixed to the entry valve and further the transfer line which is predominantly horizontal in that it is at an angle of inclination from the horizontal which is less than 45 degrees.
 20. The multi-reactor system of claim 19, wherein the angle of inclination from the horizontal of the transfer line is less than 15 degrees.
 21. The multi-reactor system of claim 20, wherein the inclination from the horizontal of the transfer line is less than 5 degrees.
 22. The multi-reactor system of claim 21, wherein the inclination from the horizontal of the transfer line is approximately 0 degrees.
 23. The multi-reactor system of claim 19, wherein the perpendicular distance between the settling leg outlet end and the entry valve inlet end is smaller than the horizontal distance between the settling leg outlet end and the entry valve inlet end.
 24. The multi-reactor system of claim 19 wherein the transfer line further comprises a means for controlling temperature.
 25. The multi-reactor system of claim 19 wherein the transfer line further comprises a means for controlling flow.
 26. The multi-reactor system of claim 19 wherein the transfer line further comprises a means for controlling pressure.
 27. The multi-reactor system of claim 19 wherein the first and second reactors are loop reactors.
 28. The multi-reactor system of claim 19 wherein both the first and second reactors each further comprise an axial flow pump.
 29. The multi-reactor system of claim 19 wherein the transfer line inlet end is affixed to the settling line outlet end by means of a three-way valve.
 30. The multi-reactor system of claim 19 wherein the isolation valve is a ball valve. 